That didn’t sound very noble to me.
It wasn’t much by racehorse standards, but it was just about the best the age had to offer. It must have been good enough, because in these early days dawn horses and their other odd-toed cousins (precursors to modern tapirs and rhinos) were very common—much more so than the even-toed artiodactyls.
There are a lot of other ways in which these earliest horses do not seem horselike. Their necks were longish, but not long. They would have had to almost kneel in order to graze on grass, had there been much. Their necks were set at a low angle so that, had they yearned to look as proud as the Vogelherd horse, they wouldn’t have been able to.
Their heads, too, were sort of horselike—but not very. The face was somewhat elongated, but still short by modern standards. The eyes sat toward the middle of the skull rather than closer to the ears. Modern horses enjoy nearly 360-degree vision, so that they can see what’s behind them as well as what’s ahead of them. (This is why carriage horses often wear blinkers; the glistening “predator” carriage following so closely on their heels terrifies them.) Dawn horses had vision that was much more limited.
Dawn horses also had padded feet, like cats and dogs. But at the end of each toe was a delicate little proto-hoof, a fingernail-like structure that protectively wrapped around each toe pad. This proto-hoof was too thin to bear the weight of the animal. Instead, the padded toes splayed out so that the pads rather than the hoofs bore the animal’s weight. Just as the feet of modern moose splay out so the huge, heavy animals can navigate swamps, the earliest horse toes provided wide footing, so the dawn horses wouldn’t get sucked into the swamps where they lived.
This is an interesting point: horses originally evolved to live not on hard, dry land, as in modern Wyoming, but in muddy, wet regions, maybe something like the jungle regions that tapirs, close cousins of horses, still inhabit today. Perhaps this muck-oriented deep-time history helps explain why horses can live today in regions such as the Camargue or on Atlantic coastal islands.
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Most of us wouldn’t easily recognize dawn horses as being ancestors of our modern horses, but what about the first euprimates, the little creatures who kept the horses company? We laypeople may not easily see “horse” in the dawn horse fossils, but it’s easy to recognize Polecat Bench primates as our ancient kind. We don’t need to have the similarities between their skeletons and our own pointed out to us in museums. The relationship is pretty obvious. In The Beginning of the Age of Mammals, the paleontologist Ken Rose compared these early primates, already adapted for leaping and grasping, to today’s bush babies. The euprimates weighed only a few ounces and had teeth smaller than grains of rice. Their extravagant tails likely provided balance as they moved from tree branch to tree branch. They already had proportionally larger brains than most other mammals.
While the dawn horse’s skull was slightly elongated, the face of the primate was shortened. The eyes of these primates had moved slightly forward on their faces, providing the beginnings of binocular vision. The ends of their forelimbs were no longer paw-like, but had become hand-like. The grasping thumb of which we humans are so proud was already evolving and appeared in very primitive form. It’s not as if the future of the primate was sealed—evolution still had many cards to play—but we can easily see in these early primate skeletons our own ancestors.
* * *
Although the earliest known horse fossil comes from Polecat Bench, the first named dawn horse fossil was found in England in the 1830s. Ironically, the fossil was found by Richard Owen, a respected scientist who would become an outspoken critic of Charles Darwin. Owen did not recognize the fossil he described as an early horse. Instead, he named it Hyracotherium and suggested it might have been distantly related to the modern rabbit.
Because of Owen’s misinterpretation, when numerous early horse fossils, recognized as such, started turning up in North America, scientists gave them a different name—Eohippus. The connection between the North American horses and Owen’s Hyracotherium was not made for quite a while, since cross-Atlantic communications were limited. The question of nomenclature has yet to be settled even today: some scientists believe that Eohippus and Hyracotherium are the same animal, while others do not. These disagreements point out how closely related so many of these animals were during the early Eocene. To mistake a twenty-first-century horse for a twenty-first-century rabbit is inconceivable, but in the early Eocene, the many groups of mammals had only begun to differentiate. Many—horses and rabbits, for example—look quite similar.
Additionally, scientists in the nineteenth century fought very bitter battles with each other over horse evolution in general. Paleontological disagreements could be nasty, and understanding the evolution of the horse was particularly vexing to European researchers. There were many horse fossils in European rocks (the later horses were more easily recognized than the dawn horses), but, oddly, these fossil horses only turned up in certain layers of time. In the Eocene epoch, which ended about 34 million years ago, Europeans found many small horses. Then, for quite a while, horses seemed to be absent from Europe. Rock layers from 10 million years ago yielded horses again. Lots of horses. And this time, they were much larger and unmistakably horselike, even though they still had three toes. Then, finally, one-toed horses turned up in great abundance.
The erratic behavior of horses over time gave Charles Darwin a serious headache. For Darwin, the process of evolution was gentle and even-paced, like a soft English summer rain. Darwin didn’t much care for perturbation—when the stress was too great he retired to his favorite health spa—and his theory of change did not include sudden heat spikes or asteroid impacts or mammalian land grabs. These discoveries came long after his death. In Darwin’s society, God would not make an undependable planet. The fact that he was claiming that life changed in a world that most other people believed to be completely static was enough controversy for him; the concept of sudden spurts of change was way beyond what he was prepared to think about.
And yet—here were these crazy horse fossils. There just seemed to be no continuity over time. Horses appeared and disappeared in Europe with an absolutely rude abruptness, presenting a kind of magical “now-you-see-’em, now-you-don’t” aspect of which Darwin thoroughly disapproved. It wasn’t just that the horses behaved oddly across the ages. It was that when they reappeared after long absences of millions of years, they were different.
Different not in just small ways, as the Sable Island horses have legs a bit different from the legs of horses who run on the open plain, but in seemingly huge ways. First they were small, then they were large. First they had four toes on their front feet, then suddenly three toes—and then only one toe. Darwin was thinking about all this long before we knew about genetics or DNA, and from the European point of view, the evolution of horses flew in the face of logic. It seemed downright flaky.
Even worse, the apparent lack of steady steps leading from one version of the horse to another provided fodder for his enemies, including Owen. Darwin and other researchers didn’t know that there were large pieces of the horse’s evolutionary puzzle yet to be discovered, and no one ever considered that those puzzles lay in the New World. When Europeans first sailed into the Western Hemisphere, there were no horses to be found. None at all. Not in North America. Not in South America. Not on the plains and not in the mountains. Scientists, who then knew nothing about plate tectonics and the ever-widening Atlantic Ocean, simply assumed that horses were Old World animals and had never lived in the Western Hemisphere.
As a young man, Darwin had found an important clue to this mystery, but he didn’t entirely understand the clue’s significance. During a several-year voyage on a British research ship, The Beagle, he had taken a side journey high into the Andes Mountains in Chile. There, of all places, he found a fossil tooth that clearly came from a horse. So what was the deal? How did that tooth get to the top of those mountains? He had no clear answer. In Chile, he had exp
erienced an earthquake which, he could see for himself, raised the level of some land by several feet, and he did understand that mountains could be created by such forces. But what was a horse, seemingly an Old World animal, doing there?
Further piquing his curiosity was another odd clue: the horses brought to the New World in the 1500s by Europeans lived long and prospered—on both the South American pampas and on the North American plains. A few escaped horses burgeoned into a fabulously prolific Western Hemisphere population. In only a century, the hundreds of imported horses quickly numbered in the tens of thousands. They seemed to be perfectly at home.
Darwin was confused. All the evidence—(1) the horse tooth high in the Andes, (2) the plentitude of horses in certain layers of European rock but not in others, and (3) the absence of living horses in the Western Hemisphere before the explorers arrived, followed by their proliferation on the pampas and plains—gnawed at him. The world just wasn’t supposed to be so unstable, but the horse fossils found in the European rock record seemed to tell a different story: Here today, gone tomorrow. It was one thing to tout a theory of soothingly logical long-term change and quite another to claim that the natural world could change rather quickly.
This was where things stood in 1877, when the English genius Thomas Henry Huxley, supporter of Charles Darwin and archenemy of Richard Owen, arrived in New York City to present a lecture. Prior to his presentation, Huxley went to Connecticut to visit the paleontologist O. C. Marsh at Yale University. A fanatical collector of fossils, Marsh had hundreds of ancient horse bones gleaned from the sands of Wyoming and elsewhere in the American West.
Marsh laid out for Huxley a whole progression of horse fossils, including many fossils that did not exist in Europe. Huxley saw exactly what Darwin had been yearning to see: Marsh’s arrangement demonstrated that the horse’s leg had indeed evolved over millions of years. Marsh showed Huxley examples of various steps along the way: horse fossils with four front toes, then a slightly later horse with three front toes, each nearly the same size, then a later horse still with three toes but with one very large middle toe and two smaller side toes, then an even later horse with a very large middle toe and two side toes so small that Marsh thought (mistakenly) that the toes were useless. Then, finally, there was a horse with only one toe.
Logic at last! Order restored to Darwin’s universe! Huxley, elated, sent the headlines to Darwin. To Darwin, evolution was about the “ascent of life,” and horse evolution now seemed to prove him correct. Horses had indeed started off as small and insignificant animals and then had, by seemingly slow and steady steps, become what they “should” be—magnificent and powerful. There was both organization and—that best of all possible Victorian nouns—progress.
The mistake the scientists had made was in assuming that horses had evolved in the Old World—a mistake that was reasonable, given that there were no horses alive in the New World when Europeans arrived. Marsh’s research made clear that horse evolution had occurred primarily in North America, rather than in Europe. The episodic appearance of horses in the Old World rock record was due to the fact that only certain New World horse species managed to find their way over to the Old World.
Marsh was also delighted. As a student studying in Europe, he had been taught that “the horse was a gift from the Old World to the New.” Now Marsh showed that the opposite was true. The horse, it turned out, was a gift to the Old World from the New. For him, it was a matter of hemispheric pride.
But wait, there’s more, Marsh told Huxley during their meeting at Yale. Marsh had an early primate fossil—also found in the American West.
For Huxley, the truth dawned: horses and primates had been partners for much longer than anyone had realized. Huxley sketched a humorous cartoon that showed an imaginary Eohomo riding an imaginary Eohippus and gave it to Marsh.
Huxley’s humorous depiction of Eohomo and Eohippus (Courtesy of the Peabody Museum of Natural History, Yale University, New Haven, Connecticut)
Long before I met Ransom and Preator and Kelley, pondering the surprising importance of horses to the science of evolution, I visited Chris Norris, the paleontologist who coined the phrase “asteroid porn.” Norris is keeper of Marsh’s horse bones at Yale’s Peabody Museum of Natural History. I asked him why horses had always been at the center of the discussion of evolutionary innovation.
“To understand change,” Norris answered, “you need a good fossil record—and horses are extremely abundant. It’s possible to look at their history over time in ways that aren’t necessarily possible with other animals.”
You can also do this with seashells, he continued, but the story of shells lacks a certain panache. People aren’t readily struck by the drama of changing shell shapes. The change in the number of toes on a horse’s foot, on the other hand, is pretty easy to see.
“Shells can’t tell you a compelling tale in the way that horse fossils can,” Norris said. “Horses are iconic and accessible.”
Then he added: “Horses can tell you a story.”
Of course, the story as understood by Darwin, Marsh, and Huxley was only partly correct. For them, the change in the number of toes on a horse’s foot represented a lovely Victorian tale of perfection. To them, horses “should” have had only one toe—one hoof per leg—and a jaw full of thoroughly efficient grinding teeth that allowed them to eat mouthful after mouthful of delicious hay.
The Victorians saw the early dawn horses as inevitably leading to their own modern, majestic animals. In their view, horses were first small and innocuous, then “progressed” over millions of years until the best of all possible horses had evolved to live in the best of all possible worlds with the best of all possible primates—us. For the Victorians, evolution was a very directed experience. The end result was foreordained. There was no meandering down paleontologically convoluted mazelike paths that eventually led to dead ends.
Today we know that the story told by horses is not a tale of journeying toward the “perfect,” but about the miracle of change on a changing planet. In Reading the Rocks, the geologist Marcia Bjornerud explains this idea a bit more formally: “Natural systems are remarkably robust precisely because no regime is permanent and no equilibrium is absolute.” The story of horses as we understand it today is every bit as good as Charles Darwin’s story, but it’s a story of process rather than of progress: The Sable Island horses have developed goatlike legs because of their unique living circumstances, and not because they are moving toward a particular, ultimate goatlike destiny. The tiny dawn horses changed over time not because bigger was better, or because faster was better, but because the world around them changed. Tectonic plates collided. Ocean currents shifted. Mountains rose. Mountains eroded. The world became hot. The world became cold. Given this kind of geological and meteorological pandemonium, had the dawn horses been “immutable”—not able to evolve—horses would have died out quite quickly.
Our planet seethes with energy. For life to survive, it must keep up with the planet’s convulsions, both abrupt and gentle—and the thrilling truth is that we’ve been able to do this. Horses and humans are survivors. Polecat Bench has provided an outstanding and entirely unexpected example of this truth.
For years, Philip Gingerich held the record of having found the oldest known horse fossil, discovered at the bench. But recently, other paleontologists found a dawn horse fossil that was just a bit older than the fossil found by Gingerich. Interestingly, this fossil was not only older—but bigger. This slightly older and slightly bigger animal lived in a world that was also slightly more lush. Then, as the heat rose at the beginning of the sudden spike, the world of Polecat Bench dried up. Plant life responded by changing. Horses responded to the shift in plant life by becoming smaller. Gingerich told me his horse fossil was about the size of his Siamese cat. The older, more recently found fossil is about the size of a small dog.
Gingerich has other evidence that horses made changes according to the environment in which they liv
ed. We know that horses were even then quite gregarious, since their fossils are often found in groups, but their social arrangements depended in part on whether they lived in thickly forested areas or in more open woodlands. Gingerich found that fossils of males and females living in open woodlands showed clear size differences, with the males being about 15 percent larger than the females. Dawn horses living in thickly forested regions were about the same size whether male or female. Gingerich concluded that mares living in open areas gathered into groups and that males had to fight each other in order to be close to these mare groups. Mares living in more thickly forested areas were more independent of each other, and males had more opportunities to mate without needing to fight. Therefore, a larger size was not necessary.
However, some traits are more flexible than others. The foot bones of the Sable Island horses were able to change rather quickly, but teeth, it seems, usually change rather slowly. When we watch horses today, we marvel at their hoofs, at their powerful necks, at their ability to stand on their hind legs. We rarely look at their teeth. But it’s modern horses’ teeth that make it possible for them to live in so many different places and to survive by eating gorse or ultra-sandy beach grass. Dawn horses would probably not have been able to survive for long on the food the Sable Island horses eat.
Modern horses have fabulous teeth, teeth that last a lifetime, teeth that are four and five inches long and that are buried so deeply in the jaw that most of us have only a vague idea of how long, powerful, and efficient they are. I was taught as a child, as were many of my equestrian friends, that horse teeth literally grow constantly over the horse’s lifetime.
This is not the case. Just like us, horses have fully formed teeth by the time they are young adults. But while our teeth emerge fully in childhood and then (hopefully) stay pretty much where they are for a lifetime, horses’ teeth do not. With horses, these fully formed adult teeth emerge from below the horse’s gum line much more slowly than do ours. The process of emergence may sometimes last as long as twenty years for horses.
The Horse Page 7